The bandwidth potentially available in optical fiber communication systems is known to be extremely large. The exploitation of this bandwidth usually requires communication systems designed to carry information in channels closely spaced in optical wavelength or frequency. The use of wavelength division multiplexing (WDM) and demultiplexing (WDD) is often proposed. WDM/WDD provides a convenient, flexible method for integrating diverse digital and analogue signals onto a distribution network and is also a means for selecting different groups of subscribers on the network. The practical implementation of WDM/WDD requires wavelength selective optical transmission devices such as filters and wavelength selective taps (or combiners). The filter transmits only a narrow band of optical frequencies (wavelengths), on the order of a gigahertz or less. The filters are useful as a means of selecting among WDM channels and also for reducing shot noise levels at the detector by removing out-of-band signals. The wavelength selective tap adds or extracts light of one or more wavelengths to or from light containing a number of other different discrete wavelengths that are assumed to be already propagating in a fiber.
A variety of techniques have been proposed for the design of narrow band filters. Filters using gratings are described in Opt. Lett., 1978, No. 3, pp 66-68 "Narrowband Bragg Reflectors in Optical Fibers" by B. S. Kawasaki et al., Electron Lett., 1985, No. 21, pp 789-799, "Narrowband Position-tuned Multilayer Interference Filter for Use in Single-mode Fiber Systems" by P. H. Lissberger et al teaches filters with dielectric multilayer films. Electron. Lett., 1985, No. 21, pp 248-249, "All-fiber Wavelength Filters using Concatenated Fused-taper Couplers" by M. S. Yataki et al, on the other hand, uses optical couplers for filters. Filters using optical interferometers and polarizing devices are described in Electron. Lett., 1985, No. 21, pp. 387-389, "Tunable Optical Multi/Demultiplexer for Optical FDM Transmission Systems" by K. Inoue et al and Opt. Lett., 1984, No. 9, pp 300-302, "In-line Fiber-polarization-rocking Rotator and Filter" by R. H. Stolen et al.
In practise, the filters that have the narrowest bandwidth are of the reflection grating type or the interferometer type. The above-referenced article by Kawasaki et al reports 50 MHz FWHM reflection filters with extremely large free spectral range determined by the harmonics of the fundamental Bragg frequency of the grating.